Electromagnetic interference (EMI) is a disturbance to the electric field due to either electromagnetic induction or electromagnetic radiation emitted from an external source. While EMI exists across the entire electromagnetic spectrum, from direct current (dc) electricity at less than one hertz (Hz) to gamma rays above 1E20 Hz, the great majority of EMI problems are limited to that part of the spectrum between 25 kHz and 10 GHz. This portion is known as the radio frequency interference (RFI) area and covers radio and audio frequencies. The acronym EMI is generally used to represent both EMI and RFI. Radio frequency interference is also described as any undesirable electrical energy with content within the frequency range dedicated to radio frequency transmission. Radiated RFI is most often found in the frequency range from 30 MHz to 10 GHz. These may be transient, continuous or intermittent in occurrence. External sources could be communication and radar transmitters, electric switch contacts, computers, voltage regulators, pulse generators, arc/vapor lamps, intermittent ground connections, solar noise, lightening electromagnetic pulses.
EMI affects the ability of high-performance electronic devices to maintain signal integrity in the time domain and for power integrity in the frequency domain. For integrated circuits, it is RF frequency that generally matters the most for mobile devices. Electromagnetic radiation generated by one electronic RF device may negatively affect other, similar, electronic devices such as cell phones, radios. For example, when a cell phone is ON, a great deal of power is transmitted. Such power interferes with RF frequencies of other devices. EMI/RFI shielding is necessary in telecomm because radio transmissions can hamper the reception of a signal by a recipient if the signals are near the same frequency. EMI/RFI shielding may prevent incorrect frequencies from interfering with a device. Another example where electromagnetic radiation is a concern is in a hospital. In a hospital, many forms of medical equipment generally must meet standards set by the Food and Drug Administration (FDA) to limit machinery from being affected by cell phones, personal digital assistants, or other electronic devices. EMI/RFI shielding helps to make such protections possible.
Bumpless build-up layer (BBUL) packaging technology is based on coreless architecture in the sense that a BBUL package or panel is separated from a core on which it is formed. The core is made up of pre-peg material, inner copper foil and outer copper foil. In vacuum based architecture, inner and outer copper foils are in contact due to vacuum created between them. In adhesive based architectures, the inner and outer copper foils are held together by a low peel strength grade adhesive between them. Core material (pre-peg) adheres to inner and outer copper foil. In order to get a coreless package, build layers are built on both sides of the core. Build-up layers are constructed by laminating, curing, drilling and desmearing dry film dielectric layers followed by semi-additive plating (SAP) process. After the desired build-up layers are laminated, the panel is routed along designated areas. The routing results in separation of the outer copper foil from the inner copper foil. Thus, the core is separated from the package(s) or panel(s). The exposed outer copper foil is then etched and any etch stop material is then removed. Thus, the final package consists of only build-up layers and embedded die or dice. Such a BBUL package is now ready for connection to a motherboard.
BBUL package is targeted for low Z height products such as mobile chips/devices. Some of these chips operate at RF frequencies and are bound to face the issue of EMI/RFI.